1. Field of the Invention
The present invention relates generally to a work roll for a rolling mill, such as cold rolling mill. More specifically, the invention relates to a work roll which has a geometrically patterned uneven dulled section with controlled roughness, to be used as a temper roll in a rolling process for producing a metal strip, sheet or plate with an improved property for coating with a coat, such as paint, enamel and so forth. Further particularly, the invention relates to a work roll suitable for production of metal strip, sheet or plate suitable to be used as painted outer panels for automotive vehicle, decorative colored panel for electric appliance and so forth.
2. Description of the Background Art
As a typical example of the painted metal sheet, the cold rolled thin steel sheet is usually produced by subjecting the cold rolled steel sheet to degreasing, annealing and temper rolling in order. In this case, the temper rolling is to improve the galling resistance in the press forming by conducting a light rolling through work rolls having a dulled surface to give a proper surface roughness to the steel sheet surface.
When such a metal plate, panel or sheet is used for a vehicular panel, particularly for a vehicular outer panel, the finish feeling after painting is a very important factor for evaluation of the vehicle per se since the external appearance of the vehicular body can be directly appealing to the customer. There are various factors for determining the quality off the painted metal sheet, panel or plate. Among the various factors, it is considered as particularly important to have a glossiness lessening any irregular reflection on the painted surface and an image clarity defining few image strains. In general the combination of the glossiness and the image clarity is referred to as "distinctness of image".
It is known that the distinctness of image on the painted surface is determined depending upon the kind of paint and the painting process but is strongly influenced by the roughness of the surface of the material metal sheet, panel or plate. Hereafter, the word "metal sheet" is used for representing various forms of metal products, including metal strip, metal panel, metal plate and so forth. Namely, when the ratio of the flat section occupied in the steel sheet surface is small and the uneveness is great, the ratio of the flat section occupied in the painted surface becomes smaller and the uneveness becomes larger, and consequently an irregular reflection of light is caused to degrade the glossiness and the image clarity, which lowers image distinctness.
In general, the roughness of the metal sheet surface is generally represented by a center-line average roughness Ra. Further, it is well known that as the center-line average roughness Ra becomes larger, the magnitude of height difference between crest and portions of the roughness becomes greater and hence the uneveness of the painted surface becomes greater, which degrades image distinctness.
When the metal sheet is subject to a temper rolling process with a working roll dulled through the conventional shot blasting process or the discharge working process, it exhibits a rough surface composed of irregularly patterned uneven dulled portions, i.e. irregularly arranged crests and concavities, as set forth above, wherein the flat section is a very small proportion of the surface area. When paint is applied to such a metal sheet, the ratio of the flat portion occupied in the painted area becomes small since the coating is formed along the surface configuration.
In order to improve the problems in the prior art set forth above, there has been proposed a surface treatment process for the work roll by means of a laser beam. Such laser beam surface treatment processes for work rolls have been disclosed in the Japanese Patent First (unexamined) Publication (Tokkai) No. Shows 56-160892, the Japanese Patent Second (examined) Publication (Tokko) No. Showa 58-22587, the Japanese Patent First Publication (Tokkai) No. Showa 54-61043, and the Japanese Patent First Publication (Tokkai) No. Showa 55-94790, for example. However, such prior proposed processes are not always successful to provide a satisfactory property for the work roll surface. In one problem encountered in the prior proposed processes, the treated surface property of the work roll tends to fluctuate depending on the condition of the work roll per se. This means, in a certain work roll condition, the property of the work roll surface obtained by laser beam surface treatment tends to be inapplicable for temper rolling of this type.